I really haven't changed my eating/drinking/exercise habits any. I did turn 30 about 2 years ago, and I wonder if it's just a metabolism slowing down kind of thing? The funny thing is, I don't look really fat. Honestly. I know a gain of 25 lbs has to show up somewhere, but I don't have stomach rolls or anything. Just a little wider and softer than before.

Are you sure you didn't change your activity level? Consider what you do when you are not doing things that are normally viewed as "exercise". For example, did you change from a more active to a less active job, or a commute that required more walking to less walking? Did you used to live in a place where you had to walk up the stairs and now you don't?

Getting wider (presumably in the waist) and softer is not a good thing, since it indicates that at least some of the gained weight is fat, rather than (healthy) muscle. Although if you do not have obvious fat rolls, you may have started out with a greater than average amount of muscle. Do you know or have you been tracking your body fat percentage, or some proxy like waist size?

What is normally seen as age-related metabolism slowdown is in part due to loss of muscle mass, often due to people becoming less active as they age.

As others have suggested, thyroid irregularities are often blamed for metabolism slowdown.